test: aggiungi suite di test Flutter

Struttura specchia lib/features/:
- test/features/meal_planner/ — unit (DayPlan, MealPlan, provider) + widget (MealCard)
- test/features/converter/ — unit (ConversionEntry, formule rawToCooked/cookedToRaw)
- test/features/shopping_list/ — unit (ShoppingItem, provider) + widget (ShoppingItemTile)
- test/helpers/pump_app.dart — estensione PumpApp condivisa tra widget test
- integration_test/app_test.dart — e2e navigazione, shopping list, convertitore, meal planner

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

test/helpers/pump_app.dart

@@ -0,0 +1,9 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:biteplan/core/theme/app_theme.dart';
+
+extension PumpApp on WidgetTester {
+  Future<void> pumpApp(Widget widget) => pumpWidget(
+        MaterialApp(theme: AppTheme.light(), home: widget),
+      );
+}